The present invention relates generally to wireless telecommunications networks, and more particularly to the minimization of facsimile image corruption in wireless networks due to channel delays.
Transmitting facsimile image data over wireless communication links such as CDMA links is now commonplace. When this type of data is sent over a noisy transmission channel, such as a CDMA channel with a high frame erasure rate (FER), air interface protocols, such as TCP/IP/PPP protocols, will cause data frames transmitted from a transmit side facsimile machine and not received by a receive side facsimile machine to be re-transmitted. Such data re-transmission inherently delays the transmission of subsequent data frames. If the cumulative delay caused by such re-transmissions becomes too large, the resulting time shift between the transmit and receive side facsimile machines will cause the transmit side machine to time out, disconnect the call, and report a transmission error.
One conventional solution to prevent a transmitting facsimile machine from timing out based on the above-described situation is to delete a predetermined number of lines from a queue associated with the transmitting facsimile machine, such as a transmission queue in a transmit side inter-working unit (IWU), with the predetermined number of lines being estimated to compensate for the time delay. However, the time shift in such an approach can only be estimated based on the number of data frames that have been re-transmitted. This is because the processing mechanism, which is typically either a base transceiver site, a fixed wireless terminal (FWT) or a central base site controller (CSBC), responsible for determining the estimated delay cannot determine the length of other system delays, as it has no way of determining whether the facsimile transmission link is a mobile-to-land, land-to-mobile or mobile-to-mobile link. In addition, the re-transmission delay could be longer than a delay corresponding to the total number of data lines capable of being stored in the transmission queue.
For example, in a mobile-to-mobile call in which the FER at the receive side facsimile machine is the dominant delay factor, the system infrastructure executes the majority of re-transmissions and therefore the facsimile scan lines must be deleted at the infrastructure queue (in the IWU). However, in a mobile-to-land call, the infrastructure queue is not a factor, as there is no air interface to cause the infrastructure queue to become congested. Therefore, in a mobile-to-land call, the mobile (FWT) is the only device capable of decreasing the time delay.
Each IWU and FWT can act autonomously based on their own time delay perspective in all types of calls, such as mobile-to-mobile, land-to-mobile, and mobile-to-land calls. Techniques in which blank lines are inserted and deleted to compensate for timing delays at the transmitting facsimile machine are known. However, if data being transmitted consists of a document having no blank lines, no compensation for timing delays would occur, therefore potentially causing the sending facsimile machine to time out and disconnect the call.